Image forming devices normally have a basic structure comprising a photosensitive drum, on the circumferential face of which an electrostatic latent image is formed based on image information from a document image which is read and/or electronically transmitted thereto, a developing device for forming a toner image on the circumferential face of this photosensitive drum by supplying toner to this circumferential face, a transport device for transporting and transferring the toner image that has been formed on the circumferential face of the photosensitive drum by the developing device to paper, and a fixing device for performing a fixing process on the toner image that has been transferred onto the paper.
The fixing device comprises a fixing roller provided with a heating means for generating heat by passing electric current therethrough, and a pressure roller disposed so that the circumferential face thereof faces and presses against the circumferential face of this fixing roller, the process of fixing the toner image on the paper being performed by heating the paper on which the toner image has been transferred with the fixing roller while causing the paper to pass through a nip formed between the fixing roller and the pressure roller, both of which are rotating.
FIG. 5 is an end-on perspective view showing one example of a conventional fixing roller member 100; as shown in this figure, the conventional fixing roller member 100 comprises a cylindrical shaft (fixing roller axle) 101; a retaining ring 103, in the form of a C-ring, which engages in an annular groove 102 that is practiced on the outer circumferential face of the end of this axle member 101; a heat resistant bushing 104, which is made of synthetic resin and fits rotatably on the cylindrical shaft 101, to the inside of this retaining ring 103; a bearing 105, having an inner ring 105a, which fits on this heat resistant bushing 104; and a fixing roller 106, which is formed integrally with the cylindrical shaft 101 at a position to the inside of the bearing 105 (which is to say at the center of the cylindrical shaft 101, in the lengthwise direction thereof). The outer rings 105b of the bearings 105 are supported by side walls of a fixing device housing, which is not shown in the drawing, whereby this fixing roller member 100 can rotate in unison with the rotation of the cylindrical shaft 101, when driven by a pressure roller, which is not shown in the drawing, within this housing,.
Here, in conventional developing devices having a fixing roller member 100 such as described above, there was a problem in that the various parts thereof expand as a result of heating by the heating means provided within the cylindrical shaft 101 and, as the coefficients of expansion and the thermal capacity of the various parts differ from one to the other, it may happen that, depending on circumstances, the internal diameter of the heat resistant bushing 104 becomes larger than the external diameter of the cylindrical shaft 101, and under these circumstances, it is possible that, while the retaining ring 103 rotates in unison with the cylindrical shaft 101, the heat resistant bushing 104 does not rotate.
Then, if the retaining ring 103 rotates while the heat resistant bushing 104 is not rotating, the end of the C-ring shaped retaining ring 103 cuts the surface of the heat resistant bushing 104, which is made from synthetic resin, as a result of which, because of the increased internal diameter of the heat resistant bushing 104, the heat resistant bushing 104 comes off from the cylindrical shaft of 101 (which is to say that the fixing roller 106 falls out of the bearing 105).
In the foregoing, conventional problems were described for a situation in which the heat resistant bushing 104 ceases to rotate readily in unison with the cylindrical shaft 101, but similar problems arise when the retaining ring 103 ceases to rotate readily in unison with the cylindrical shaft 101.
The present invention is directed at resolving such problems, and an object thereof is to provide a fixing device capable of reliably preventing the heat resistant bushing from falling off of the fixing roller shaft by ensuring that the bushing and the retaining ring rotate in unison under all circumstances.